The complement system is composed of nearly 50 individual proteins that functions as a part of the innate immune system providing the initial phase of host defense, opsonization of foreign material, and tissue homeostasis. (Ricklin D., 2010, Complement: a Key system for immune surveillance and homeostasis. Nature: Immunology, 785-795) The complement system is found in all multicellular organism and phylogenetically predates the formation of the adaptive immune system (Zarkadis I. K., 2001 Phylogenetic aspects of the complement system. Development and Comparative Immunology, 745-762.). Activation of the complement system occurs along three primary pathways: classical, lectin and alternative pathways. FIG. 1 shows a schematic representation of the three primary complement pathways. See also, Donoso, et al., “The Role of Inflammation in the Pathogenesis of Age-related Macular Degeneration”, Survey of Ophthalmology, Vol. 51, No. 2, March-April 2006.
During the activation process sequential protein-protein interactions and proteolytic activity leads to the generation of the C3 and C5 convertases. These convertases are responsible for producing complement activation split products that represent the effector molecules of the complement cascade important for opsonization, generation of anaphylatoxins, and the formation of the membrane attack complex (MAC). The latter of these is essential for the lytic activity of the complement cascade (Ricklin D., 2010). Under normal conditions activation of the complement cascades provides defense against pathogenic bacterial, viruses as well as clearance of diseased and injured tissue. Normally, the formation of MAC does not affect surrounding tissue due to the presence of cell surface and soluble regulatory components which include CFH, CFH related proteins, C4BP, CD46, CD55, CD59 and complement factor I (CFI). However, when excess activation occurs or when there is a failure in complement regulatory components, both acute and chronic disease states are induced. Examples in which uncontrolled complement activation is recognized as causative to human pathologies include: Glomerulonephritis, Systemic Lupus Erythematosus, Paroxysmal Nocturnal Hemoglobinuria, Alzheimer's, Hereditary Angioedema, Myasthenia Gravis and Age-related Macular Degeneration (AMD) (Ricklin & Lambris, 2013, Complement in Immune and inflammatory Disorders: Pathological Mechanisms. Journal of Immunology, 3831-3838).
C5 is a 190 kDa protein comprising two polypeptide chains (α, 115 kDa and β, 75 kDa) that are linked together by disulfide bonds. C5 convertase cleaves at an arginine residue 75 amino acids downstream from the C5 α-chain N terminus generating the 7.4 Kd C5a and 180 Kd C5b complement split products. The C5b component serves as the initiation component for the assembly of the membrane attack complex (MAC) through the sequential addition of C6, C7, C8 and C9. The C6-C8 subunits assemble in a 1:1 relationship to C5b while multiple C9 subunits are incorporate into the complex generating a non-specific pore in both prokaryotic and eukaryotic plasma membranes FIG. 2. See also, Bubeck D., 2014, “The making of a macromolecular machine: assembly of the membrane attack complex” Biochemistry, 53(12):1908-15. The formation of MAC on the cell surface has several consequences for the cells. At high levels the unregulated influx and efflux of solutes leads to cellular swelling and eventual cell lysis. This causes the uncontrolled release of cellular material promoting a pro-inflammatory environment and cellular loss. Formation of MAC at sublytic concentrations on the cell surface can contribute to release of pro-inflammatory and pro-angiogenic cytokines and growth factors, elevation in cellular stress and eventual necrotic cell death.
Age-related Macular Degeneration (AMD) is the leading cause of blindness in the elderly developed countries. In the US population alone the prevalence of advanced forms of AMD associated with vision loss occurs in nearly 2 million individuals. Another 7 million individuals with intermediate AMD are at a high risk for development of advanced forms of AMD. Inclusion of the European population nearly doubles the number of impacted individuals. AMD is characterized by a progressive loss of vision attributable to a para-inflammatory process causing the progressive degeneration of the neuroretina, and support tissues which include the retinal pigmented epithelium (RPE) and choriocapillaris. The majority of clinically significant vision loss occurs when the neurodegenerative changes impact the center of the retina within a highly specialized region of the eye responsible for fine visual acuity, the macula. The disease has a tremendous impact on the physical and mental health of the individual due to vision loss and increased dependence on family members to perform everyday tasks.
The deregulation of the complement system is highly correlated with the development of AMD. First, genetic mutations in over 20 genes have been correlated with a person's risk of developing AMD, accounting for an estimated 70% of total risk. (Fritsche et al., “Age related Macular Degeneration: Genetics and Biology Coming Together”, Annu Rev Genomics Hum Genet. 2014; 15:151-71). Within these 20 genes, five are complement genes, which alone account for 57% of total risk in the development of the advanced forms of AMD. In addition, AMD-related inflammation and associated deregulation of complement activity, as indicated by elevation of complement activation products in systemic circulation and in AMD tissues by histopathological analysis, occurs in the absence of known genetic polymorphisms in complement proteins. New discoveries, have highlighted the potential pathological impact of complement by the identification of and presence of the membrane attack complex in diseased tissue and in occurrence of advanced forms of AMD (Whitmore S, et al. 2014, “Complement activation and choriocapillaris loss in early AMD: Implications for pathophysiology and therapy.” Progress in Retinal and Eye Research, Dec. 5, 2014 EPub ahead of print; Nishigauchi K M, et al. 2012 “C9-R95X polymorphism in patients with neovascular age-related macular degeneration”, Jan 131; 53(1) 508-12). These results suggest the viability of blocking the final complement pathway component as a therapeutic target for treating AMD. To date most therapeutics targeting formation of MAC do so by blocking the formation of C5b the key building block required to initiate MAC formation. However, in doing so they also block formation of C5a resulting in loss of C5a functional activity that has been associated with tissues homeostasis (removal of opsonized particles), neural survival and promotion of an anti-angiogenic response. In man, this process of selectively blocking MAC formation is usually carried out by the cell surface protein CD59 which blocks MAC assembly and by the soluble factors vitronectin and clusterin. In order to mimic the natural mechanism and preserve favorable upstream activities of complement activation the current application reveals the development of a novel therapeutic monoclonal antibody that binds C5 but uniquely allows processing of the C5 molecule to C5a and C5b but inhibits formation of MAC, FIG. 2, thus preventing formation of the key pathogenic component associated with AMD. Through blocking MAC formation, while preserving key supportive ocular tissues i.e., choriocapillaris and RPE, function and survival of the neural retina, which is vital to maintaining vision will be retained.